<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <title>盒子模型</title>
    <link href="main.css" rel="stylesheet" type="text/css">
</head>
<body>
    <div class="box1">
        <div class="box2" id="box21">盒子2-1</div>
        <div class="box2" id="box22">盒子2-2</div>
        <div class="box2" id="box23">盒子2-3</div>
        <div class="clear"></div>
    </div>
</body>
</html>